Positioning systems, such as Global Navigation Satellite Systems (GNSS) (the current operational example is the US NavStar GPS system) receivers, are increasingly being integrated into battery-operated terminals (i.e., netbook personal computers and mobile wireless devices). The orbiting satellite constellations provide low-power, wideband radio signals that the receiver uses to estimate the location of the user terminal. These location estimates are typically provided to on-board applications running on a co-located processing unit. To maximize the duration between battery recharge, fuel cell fill-up, or replacement of the power source, power consumption by the location receiver subsystem should be limited. Currently location receiver subsystems are only powered when actively required by a location-based application and even then are periodically powered down resulting in an periodic but intermittent set of location estimates.
A variety of terrestrial digital radio beacons with known signal characteristics (transmission frequency, frequency band, demodulation, framing, framing rate, bit patterns) and broadcast identifiers are available in the form of wireless local area networks (WLAN) and wide area cellular communications networks (Wireless Communications Networks (WCNs) are deployed to allow users to access said networks.